Heteroaromatic rearrangements

Full details have appeared of the behaviour, inter alia of 2,6-disubstituted aryl 2-nitrophenyl sulphides with triethyl phosphite and of 2,6-disubstituted aryl 2-azidophenyl sulphides on thermolysis, following preliminary reports (see Volume 1, p. 470). In both cases the reaction involves a novel heteroaromatic rearrangement that is interpreted in terms of the intervention of nitrene intermediates. The process is exemplified by the behaviour of 2-nitrophenyl 2,6-dimethylphenyl sulphide (40) and 2-azidophenyl 2,6-dimethylphenyl sulphide (41) (Scheme 6). The sigmatropic rearrangement of the postulated intermediate (42), which is necessary to produce the [Z>,e]azepine (44), is not allowed in the suprafacial sense hence the intervention of a biradical species (43) is considered to be more likely. 

A useful approach to the substitution of ring C—H positions lies in the activation of the heteroaromatic system by an A-oxide group, initiating a formal intramolecular redox reaction. 1-Methyllumazine 5-oxide reacts with acetic anhydride in a Katada rearrangement... 

We mention Williams work briefly here because it may also explain Blangey s observations strongly basic primary amines unequivocally form 7V-nitrosoanilinium ions in strongly acidic media. In contrast to the rate-limiting deprotonations of the less basic aromatic and heteroaromatic nitrosoamine cations discussed in this section, the TV-nitroso cation of a strongly basic amine deprotonates extremely slowly. Therefore, the nitroso rearrangement, the Fischer-Hepp reaction, competes effectively with the 7V-deprotonation. 

The role of oxazirdines as intermediates in the rearrangement of heteroaromatic N-oxides has not been fully established although in most cases the formation of photoproducts can best be rationalized in terms of such intermediates. Rearrangement appears to be singlet-derived and competes with triplet-derived deoxygenation. 

Disubstituted 4-chloro-2-cyclobutenones 75 undergo the palladium-catalyzed cross-coupling reaction with vinyl- and arylstannanes 76 or vinylzir-conium reagents to give the 4-R sa,-2-cyclobutenones 77. Without isolation, these cyclobutenones 77 are rearranged to the substituted phenols 78 on thermolysis [38], Application of this method to the stannylated heteroaromatics 79 provides a synthetic route to the aromatic benzoheterocycles 80 [39]. (Scheme 27 and 28)... 

Few examples of the preparation of six-membered heteroaromatic compounds using Fischer-type carbene complexes have been reported [224,251,381]. One intriguing pyridine synthesis, reported by de Meijere, is sketched in Figure 2.35. In this sequence a (2-aminovinyl)carbene complex first rearranges to yield a complexed 1 -azadiene, which undergoes intermolecular Diels-Alder reaction with phenylacetylene. Elimination of ethanol from the initially formed adduct leads to the final pyridine. 

C-H Insertions into vinylic C-H bonds are also a common reaction of electrophilic carbene complexes. Insertions into aromatic or heteroaromatic C-H bonds can proceed via cyclopropanation and rearrangement (Figure 4.6). 

The same activity is presented by an iridium complex [Ir(Cp )Cl2]2 that catalyses the Beckmann rearrangement of aromatic, aliphatic and heteroaromatic aldoximes 276 into the corresponding primary amide 277 in good to excellent yields (78-97%) (equation 85). 

It was again observed that rearrangement pathways comprise a substantial portion of the oxidation routes for alkylated aromatics.Since this phenomenon is mainly due to peroxy radical reactivity rather than to identity of the parent compound, it is clear that comparable rearrangements would be factors for PAHs, as well as for nitrogen-, oxygen-, and sulfur-containing heteroaromatic rings and their alkylated derivatives. 

DMAD get thermally converted to quinolones (66), through a Cope rearrangement. These quinolones give rise to tricyclic heteroaromatic compounds, namely, pyrano[3,4-6]quinoline-l,5-diones (67) and furo-[3,2-c]quinolines (68), depending on the reaction conditions (Scheme... 

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